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(HP-28S) Non-Invasive fix for dead arrow keys on clamshell calculators
12-21-2018, 05:51 AM (This post was last modified: 12-21-2018 06:34 AM by wyatt8740.)
Post: #1
(HP-28S) Non-Invasive fix for dead arrow keys on clamshell calculators
Hi. I'm not really a regular here, so I'm not sure if this would qualify as an "article" or not (which is why I'm posting it in general instead of articles), but I am a college student studying computer science who discovered the HP-28S and fell in love with it. I feel like all of the clamshells are under-represented here and elsewhere on the web – like they live in the shadow of the 48SX, which was an improvement in terms of connectivity but a step backwards in most other respects in my opinion, so I wanted to try to contribute to the meager knowledge pool I've been able to unearth for repairing them.

There are several images embedded, which you can click to view the original sizes of. If you want to tell me how badly I've done this, or what I should have done differently, or why I shouldn't have done what I did, I'm open to those comments. I am not under the delusion that this is a beautiful solution, or even a particularly good idea, but it's one that worked for me.
On to the story!

Unfortunately, my particular specimen has a problem in that it's got a crack in the case near where the lid clips to the lower body, and shortly after obtaining it I discovered that the arrow keys were causing trouble: first operating erratically, and then not at all unless I applied pressure to the case in the vicinity of the crack. I did, however, get one with an intact battery door, so in that regard at least I am blessed.

This is the best pre-repair photo I can find of my 28S (next to a 48SX I later purchased, but which I don't like as much as the 28 for programming on. I didn't get a 48GX because they cost so much more than a 48SX for a modern, loan-debt-ridden college student).
[Image: rDR0LKTh.jpg]

I really didn't feel like I had the time or motivation to do the "proper" fix of taking it entirely apart and repairing the heat stakes, so as a spot fix—which I hopefully won't regret later—I decided to try something else:

[Image: KxRnc3Xh.jpg]
I used it to both fill in the crack in the LCD's "bezel" area. More importantly (in terms of fixing the buttons' functionality) I also used it adhere the part of the outer side of the "bezel" which sinks out of sight behind the lower shell, to the lower shell.

I used one of those plastic tools you always see in those ifixit guides (I think they're called "spudgers"?) to pry the plastic upwards to get a surface to put the glue on, but some of it of course pressed out of the top and had to be scraped off.

I just pinched the shell together while I waited for the glue to set, making sure that I was holding it tight enough that the top row of buttons were working properly, but you should probably use a C-clamp or winch if you have one handy to CAREFULLY, GENTLY pinch the case together and hold it for longer. (Maybe put some cardboard on either side of the calc body or something, to stop it from wrecking the shell?)
Anyway, this might work better if you do that, and let the glue set for longer than I did before proceeding. I think I held it for ten to fifteen minutes, tops. Probably more like seven to ten.

Unfortunately, I had somehow forgotten the rather important fact that superglue leaves awful looking white stains around everything I have ever used it on. It didn't look too bad from a distance, but I knew it was wrong and had to go.
[Image: T7L7qtkh.jpg]
I sanded the glue down after taking those pictures, which helped a lot, but didn't entirely eliminate the stains. So I proceeded straight to my next step without taking pictures of the sanding (I used 2000 grit sandpaper, by the way). I recommend you do that to at least smooth down the glue for the next step, anyway, unless you were much better applying the glue than I was.

Anyway, the white stains were still bugging me, but the calculator worked. I couldn't stop there, though; I HAD to eliminate that awful white scourge on my vision. Maybe if I had one of those rare transparent 28S's (manufacturing prototypes, I think?) it wouldn't be so bad. Anyway, I opted to paint it. I actually got a pretty good match, but it looked even closer before it dried. So I'd suggest aiming lighter than I did. I used no-name acrylic paint – "Burnt Umber" and black, in roughly equal measure. Maybe a little more black than umber, to be truthful. The 28S is a pretty dark brown.
[Image: qtDbYCeh.jpg]
Pretty strong results for a 20-30 minute job, in my opinion. We'll see how it holds up to repeated openings/closings. If I ever have to do the complete renovation (re-staking,) I imagine I'll need to use a dremel or chisel to break the glue to get into it, so that's a possible drawback to this method.

Picture of the full calculator from the front (user perspective):
[Image: iZdXbyMh.jpg]]

Close-up from the front, showing how little of the scar (crack) remains visible after gluing, sanding, and painting:
[Image: vbWUCo8h.jpg]

"Far-down" (opposite of "close-up"):
[Image: EChppN5h.jpg]

Original for the "after" photo from the before/after comparison:
[Image: EQDDMMBh.jpg]

I hope this will give my calculator a new lease on life and let it continue doing its thing for years to come. Long live RPL!

Anyway, I welcome your criticism, questions, or whatever else now! Hopefully I'll remember to check in on this thread, since it's not a habit for me yet. I hope I'm not posting too many pictures. Thoughts?

Wyatt Ward
Computer history enthusiast
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12-23-2018, 09:54 PM
Post: #2
RE: (HP-28S) Non-Invasive fix for dead arrow keys on clamshell calculators
*Very* nice! Thanks for posting your results!
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12-28-2018, 08:12 PM
Post: #3
RE: (HP-28S) Non-Invasive fix for dead arrow keys on clamshell calculators
Great post and photos!

The 28S is my favourite HP and the keyboard feel is beautiful never bettered in my opinion. My advice to remove excess superglue is to use Turps (Paint thinner?) It worked well on my old Alienware laptop when I reglued a broken grill piece!

HP-28S (1988 US model), DM41X (2020)
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